For days, Clinton has cast Sanders as a less forceful advocate for gun control, honing in on a 2005 vote he cast that gave immunity to gun manufacturers. On Tuesday, she broadened her critique, arguing that if Sanders wouldn’t combat the National Rifle Association, he can’t be trusted to take on other special interests.

“If you’re going to go around saying you stand up to special interests, then stand up to that most powerful special interest  stand up to the gun lobby,” she said, as she accepted the backing of a major gun control advocacy group.

Clinton added: “Don’t talk to me about standing up to corporate interests and big powers. I’ve got the scars to show for it.”

Sanders campaign spokesman Michael Briggs said the senator had “spent a career standing up to powerful special interests whether they be Wall Street, big banks or Big Oil or the pharmaceutical industry, you name it.”

Briggs added, “He has also stood up to the National Rifle Association,” noting that Sanders lost a 1988 congressional race in part because he supported a ban on assault weapons.

The fresh critique marks an effort by Clinton to undermine the central argument of Sanders’ campaign that the Vermont senator is an outsider offering liberals a “political revolution.” At one point, she alluded to his mantra, telling supporters, “If that’s the kind of ‘revolution’ he’s talking about, I’m worried, folks.”

With a touch of sarcasm, Clinton derided Sanders’ plans for a single-payer Medicare-for-all system and said President Barack Obama’s work to pass an overhaul to the nation’s health care system was a major accomplishment.

“I wish that we could elect a Democrat who could wave a magic wand and say, ‘We shall do this and we shall do that.’ That ain’t the real world we’re living in,” Clinton said. In Dubuque, Clinton said she hoped that Sanders “hurries up” and releases more details of his tax plan “because you deserve to see the comparisons side-by-side.”

“It is a national disgrace that the United States is the only major country in the world that does not offer health care as a right,” Weaver said. “We need a president who will fight for the 29 million Americans without health care.”

Clinton also touted her foreign policy credentials, drawing another, more implicit, contrast with Sanders, who has made tackling economic inequality the focus of his campaign. “I’m prepared to do all parts to the job,” she said, after offering voters a detailed account of her time in the Situation Room during the killing of Osama bin Laden.

Though Clinton has the backing of Democratic leaders and top donors, polls show a tighter race in Iowa while Sanders has built a slight lead in New Hampshire, which borders his home state of Vermont. Losses in both early voting states could raise worries among Democrats about her strength against Sanders, who was relatively unknown when he started the campaign but has attracted big crowds to his rallies.

A poll released on Wednesday by Quinnipiac University showed Sanders winning 49 percent of likely Democratic caucus-goers in Iowa, compared to 44 percent for Clinton. The same survey showed Clinton leading in a 51-40 percent match-up just a month ago.

Clinton has long resisted directly targeting Sanders, fearing that such attacks would alienate his passionate supporters. Should she win the nomination, Clinton will need that kind of liberal enthusiasm to boost her to victory in a general election.

Daughter Chelsea Clinton also got in the act, telling a New Hampshire audience that Sanders would open the door for Republican governors to undermine the health care law.

Both candidates received dueling endorsements on the eve of Obama’s State of the Union address. Clinton picked up the backing of the Brady Campaign to Prevent Gun Violence  an endorsement that came just a day after she won support from leading gun control advocate Gabby Giffords, the former Arizona congresswoman, and her husband, Mark Kelly.

Dan Gross, president of the Brady Campaign, noted that the two candidates diverged on the landmark Brady handgun bill, which imposed a five-day waiting period for gun purchases. “Bernie Sanders actually voted against it. But there’s only one candidate who fought for it and that candidate is Hillary,” he said.

Sanders has expressed his support for Obama’s use of executive actions to curb gun violence and has said he would revisit his position on the liability issue. But in a Democratic forum on Monday night, he doubled down on his defense of his vote on the controversial bill.

“It’s not a mistake. Like many pieces of legislation, it is complicated,” he said. “But on the issue of guns, let me be very clear I support the president.”

Clinton was also endorsed by the United Food and Commercial Workers Union, a union of 1.3 million members.

Sanders racked up his own endorsement on Tuesday, receiving the backing of MoveOn.org, a grassroots organization that has been at the forefront of liberal causes.

He was supported by 78.6 percent of its membership in an online vote of more than 340,000 members, according to the group. Hillary Clinton received 14.6 percent and Martin O’Malley received 0.9 percent with the remaining members urging no endorsement. The group said it plans to mobilize thousands of its members in Iowa and New Hampshire on Sanders’ behalf.

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Thomas reported from Washington. Associated Press reporter Holly Ramer in Manchester, New Hampshire, and Catherine Lucey in Dubuque, Iowa, contributed to this report.